Reactively actuated acoustoelectric transducer



s. M. CHURCH 2,650,954 REACTIVELY ACTUATED ACOUSTO-ELECTRIC TRANSDUCER Sept. '1, 1953 Filed May 22, 1948 fiMpL/TUDE FEEQUENCV FQEQUENC'P INVENTOR lit.

ATTORNEY Patented Sept. 1, 1953 UNITED STATES E ATENT OFFICE REACTIVELY ACTUATED ACOUSTO- ELECTRIC TRANSDUCER 2 Claims.

This invention relates generally to a transducer and, more particularly, to a microphone or a phonograph pick-up of the type responsive to mechanical input thereto, rather than the type responsive to the rate of change of mechanical input thereto.

Numerous prior art transducers of the type responsive to mechanical input thereto have been developed, such as the crystal microphone or pick-up or condenser microphone or pick-up. Prior art condenser microphones or pick-ups have had numerous disadvantages. For one thing, since the condenser was charged to a relatively high potential through a relatively high value of resistance so that capacitative variations of said condenser occurring as a result of auditory input thereto would cause corresponding potential variations across said condenser adapted to vary the grid bias of an amplifying tube, a relatively large value of capacitance had to be used. This made it necessary that the movable pressure-responsive diaphragm forming one plate of said condenser microphone be of relatively large area and consequently large 2 mass, if sufiicient potential variations were to occur as a result of the auditory input thereto to result in a satisfactory output. Even so, a preamplifier has generally been required in such condenser microphones.

The relatively large size and mass of the prior art condenser microphone diaphragm had the very undesirable effect of greatly increasing the mass resonance ratio thereof. This resulted in the natural resonant frequency of said diaphragm occurring within the audio frequency range which it was desired to transduce. For example, in one typical prior art-type condenser microphone, a resonance peak of approximately 6 decibels occurs in the neighborhood of 3500 cycles per second antiresonance occurs in the neighborhood of the first harmonic thereof. Thus, the frequency response of such prior art condenser microphones was not uniform. Furthermore, in the attempt to raise the natural resonant frequency of such a condenser microphone diaphragm to a higher value, increased tension has been applied to the diaphragm in certain instances. This has been only partially successful in that while it raised the natural resonant frequency of such diaphragms to a higher frequency level, it did not remove the natural resonant frequency entirely from the audio frequency range. It had the undesirable effect of causing a lack of stability, because no adequate means for maintaining such extreme tension uniform over a period of time and through various temperature changes, was available.

In addition, since a relatively high potential was maintained across the diaphragm and plate of such condenser microphones, of the order of 200 volts, and since the diaphragm and plate were positioned very close together in order to effect the desired capacitive changes in response to auditory input thereto, very frequent electric arcovers or break-downs occurred Where the current passed through the air between the plate and diaphragm or vice versa. The extreme heat thus generated usually pitted or perforated the diaphragm, thus rendering it unusable. Furthermore, such prior art condenser microphones have generally been bulky and of relatively great mass, of the order of 6 or '7 lbs., in the case of a microphone used for motion picture sound recording, for example.

Moreover, such prior art microphones have generally been relatively non-linear in response and have had a relatively low signal-to-noise ratio and have been characterized by substantial intermodulation products. One reason for this is as follows. The variation in capacitance between two plane surfaces with a variation in distance between them is inversely proportional to the square of the distance. Thus, if the ratio of capacitative variation to the normal capacity is very great, considerable non-linearity results. This is even more true of a phonograph pick-up since the excursion and corresponding capacitative changes are relatively very much greater than in the case of a microphone. Furthermore, such microphones have a low frequency electrical attenuation or cut-off characteristic, necessitating the use of an equalizer.

Generally speaking, the apparatus of this invention comprises a means for generating electric oscillations of a predetermined high frequency and means for reactively modulating said high frequency oscillations within a selected low frequency range in response to mechanical input thereto.

The apparatus of the present invention has none of the disadvantages hereinbefore mentioned inherent in prior art constructions. For example, the actual capacity between the diaphragm of the present invention and the adjacent plate may be very much smaller than in prior art condenser microphones, since it is not necessary to generate a fluctuating potential thereacross in a manner corresponding to the operation of the prior art-type of condenser microphone, the condenser of the present invention acting merely to change the LC product of a tuned resonant circuit, which has coupled to it electric oscillations of a predetermined high frequency which are thus reactively modulated by said variably tuned resonant circuit. Furthermore, in the construction of the present invention, a high voltage is not carried by the capacity but rather a voltage of a relatively low order. Thus, the hereinbefore-mentioned highly undesirable condenser arcing and consequent pitting is avoided. Furthermore, since a relatively low capacity is required, the size and mass of the diaphragm may be relatively small, thus resulting in a low mass resonance ratio whereby the natural resonant frequency of said diaphragm is outside the useful audio frequency range. Therefore, the frequency response of the transducer of the present invention does not have undesirable irregularities arising from resonance eifects. Furthermore, because of the relatively low mass and size of the diaphragm, the tension applied thereto need not be excessive, and stability of such a diaphragm is easily achieved.

In addition, because of the relatively small size of the essential components, the microphone of the present invention is relatively small and light, a microphone for motion picture sound recording, for example weighing on the order of 1 lb. Furthermore, the modulator of my invention has a very high Q because of feeding into an electron tube of virtually infinite input impedance. Thus, the slope of the amplitude vs. frequency curve thereof is very steep and linear and the operating point of my apparatus is on one of the linear sides of said curve, whereby very slight capacitative variations occurring in response to auditory input to the microphone cause very large and linear amplitude variations. Thus, the non-linear response of prior art condenser microphones is avoided and, furthermore, since such a large response occurs through the use of the apparatus of the present invention, the ratio of capacitative change to the total capacity in the tuned resonant circuit may be made relatively small, thus further improving the linearity.

Since the band width passed is relatively narrow in contrast to prior art condenser microphones, random noise resulting from thermal agitation and shot effect is minimized.

A much greater signal-to-noise ratio is obtained through the use of the present invention. In the case of the hereinbefore-mentioned microphone adapted for motion picture sound recording use and weighing approximately 1 1b., a gain of approximately 25 to 30 decibels is realized over the response attained through the use of the customary condenser-type microphone intended for motion picture sound recording use and weighing approximately 6 to 7 lbs.

Furthermore, because of the increased linearity of the apparatus of the present invention, it may readily be adapted for use in phonograph pickups with a relatively undistorted output therefrom in contrast to the very distorted output of such a pick-up employing prior art condensertype construction.

With the above points in mind, it is an object of this invention to provide a new and improved condenser-type transducer of greatly improved response characteristics and greatly reduced size and mass.

It is a further object of this invention to provide a new and improved condenser-type microphone having a relatively high signal-to-noise ratio and no resonance peaks in the response characteristic.

It is a further object of this invention to provide a new and improved condenser-type microphone of very simple, cheap, small, light, foolproof construction.

It is a further object of this invention to provide a new and improved transducer reactively modulated in response to mechanical input thereto, of very simple, light, cheap, foolproof con struction having an improved signal-to-noise ratio and no resonance peaks in the response characteristic.

Other and allied objects will become apparent to those skilled in the art upon a careful examination and study of the illustrations, specification and appended claims.

To facilitate understanding, reference will be had to the following drawings, in which:

Fig. 1 is a diagrammatic, electrical, schematic view of one illustrative form of this invention.

Fig. 2 is an illustrative graph of a high Q resonant circuit such as is employed in the present invention.

Fig. 3 is an illustrative graph of a low Q resonant circuit.

Referring to Fig. 1, means for generating electric oscillations of a predetermined high frequency are provided and are indicated generally at I, which will be described more specifically hereafter. The oscillation-generating means I is coupled to a modulator, indicated generally at 2, which will be described more specifically hereinafter, and which is adapted to reactively modulate said high frequency modulations in response to mechanical input thereto, which is connected to a rectifier tube 23.

The oscillation-generating means I, in the example shown, comprises an electron tube 3 having an anode 4, a cathode 5 and a control grid 6, with the cathode connected to ground at T, the control grid 6 connected to one side of a crystal 8, and through a resistor 9 to ground at [0, the other side of the crystal 8 being connected to point I l, which is positioned between two condensers l2 and 13, forming a voltage divider connected at one end to the cathode 5 and at the other end to the anode circuit at a point indicated at M for providing feed back to the crystal 8 and grid 6. The anode 4 is connected to a tank circuit, indicated generally at (5, including a capacitor [6 shunting a coil H. The tank circuit is then connected to the point It and through a resistor H! to a suitable source of positive potential indicated at 4| the negative end of which is adapted to be connected to ground.

The modulator, indicated generally at 2, includes a parallel resonant input circuit, indicated generally at l9 and comprising an inductive coil 20, shunted by a variable capacitance 2!, adapted to be varied in response to mechanical input thereto. In the case of a microphone, the variable condenser 2| may comprise one or more condenser plates and a movable diaphragm positioned adjacent thereto adapted to move in response to auditory input thereto. The inductive coil 20 is coupled to the coil I! of the tank circuit l5 of the oscillator i. This coupling may be adjustable for optimum results. The upper end of the parallel resonant input circuit I8 is connected to the grid 22 of an infinite impedance electron tube 23, and the lower end of the parallel resonant input circuit I9 is connected to ground at 24 and is also connected to carrier frequency by-pass condenser 25, the opposite end of which is connected to the cathode 26 of the electron tube 23. The cathode 28 is connected to a point 2? which is connected to one end of a cathode resistor 28, the opposite end of which is grounded at 24. The point 27 is also connected through a carrier frequency choke coil 30 to one end of a low or audio frequency passing condenser 35, which is connected to the upper end of the primary winding 32 of a low or audio frequency output transformer, indicated generally at 33. The winding 32 is shunted by a resistor 34 for providing proper terminal impedance for the winding 32, since the resistor 28, which is effectively across the terminals of the winding 32 in parallel to the resistor 36, may be of a Very high resistance value. Therefore, the resistor 34 is employed to provide the proper impedance value across the terminals of said winding. The output or secondary winding 35 of the audio transformer 33 is adapted to deliver the low or audio frequency signal to any desired point for utilization. The anode 36 of the electron tube 23 is connected to ground at 3? by a carrier frequency by-pass condenser 38, and is connected to a suitable source of positive potential, indicated at 39, through a carrier frequency choke coil 49. The negative end of the source of positive potential, indicated at 39, is adapted to be connected to ground, as is customary.

The operation of the device may be explained as follows. The oscillation-generating means I, in the examples shown as a crystal-controlled oscillator, generates a carrier wave of a predetermined high frequency, for example, let us say 14 megacycles. The frequency stability of this signal is quite good since it is crystal controlled. The 14 megacycle signal is fed into the modulator 2 by reason of the electromagnetic coupling between the coil I! in the oscillation-generating means i and the coil 20 in the resonant input circuit is of the modulator 2. The resonant input circuit is is not tuned to 14 megacycles exactly but is tuned to a frequency slightly below or slightly above 14 megacycles, such that the amplitude of the signals is reduced to a value between the maximum and minimum values thereof on the amplitude vs. frequency curve thereof. This is best illustrated in Fig. 2, where the point as indicates the frequency to which the resonant input circuit I9 is tuned, 14 megacycles being indicated at ea. In the example illustrated, the point 12 occurs at approximately 70% of the amplitude of the point 4-3. I have found this to be a desirable operating point although I do not intend to limit myself to this figure. It can be seen, referrin to Fig. 2, that the curve on each side of the operating point 42 is virtually linear and is very steep, so that a very slight change in the tuning of the resonant input circuit l9 resulting from very slight capacitive changes of the condenser microphone 2! caused by auditory input thereto will cause a relatively large amplitude variation of the carrier wave and that such variation will be relatively linear. The tube 23 is connected to the resonant input circuit [9 in such a manner as to act as an infinite input impedance rectifier or detector tube and is adapted to demodulate and detect the modulated high frequency oscillations. Although it is connected across the resonant input circuit l9, it does not lower the Q thereof because of the virtually infinite impedance ofiered by the grid 22 and the cathode 25 to the tuned resonant circuit l9, during any portion of a cycle of operation, by virtue of the fact that the loading takes place between the cathode 26 and round 24; thus, effective isolation can be realized and no rectified carrier current flows in the tuned resonant circuit I9. It should be noted that when the tuned resonant circuit [9 is feeding unmodulated high frequency voltage oscillations to the rectifier tube 23, the peaks or maximum amplitude of the high frequency oscillations fall on the virtually linear central portion of the Ip vs. Eg curve, whereby the tube will operate class A with respect to the low frequency modulating oscillations, which will, therefore, be linearly reproduced. The cathode resistor 28 may be adjustable if desired, and a suitable current-measuring device placed in series therewith for adjusting said resistor when the tuned resonant circuit I9 is feeding unmodulated high frequency oscillations to the tube 23 for causing the tube to operate as just described. With the tube 23 operating in this manner, the resonant input circuit l 9 has a very high Q, consisting primarily of inductance and capacitance with no resistive loss components. Therefore, an amplitude vs. resonant frequency curve of the type shown in Fig. 2 is attained, which results as hereinbefore mentioned in a relatively larger output for a given acoustic input to the microphone and which further results in said output being relatively linear and free from distortion and intermodulation products and having a substantially higher signal-to-noise ratio, since the band is very high and very narrow, as shown in Fig. 2, being approximately 70 kc. for the whole curve. If in place of the above-described infinite impedance rectifier a diode detector is used, as has been common in the prior art, a very low Q results, since resistive loss components are present. Substantial current flow through the diode and the tuned resonant circuit elements would cause thermal frequency drift, which in turn would be reflected back into the tank circuit of the oscilla tion-generating means, causing a frequency and amplitude variation therein occurring at a rapid oscillatory rate, thus resulting in great distortion, since it is essential in the practice of the present invention that the amplitude and frequency of the output oscillations of the high frequency oscillation generator l remain substantially constant at all times. The use of the hereinbefore described infinite impedance rectifier avoids this undesirable effect. Furthermore, the use of such diode detector causes undesirable effects by reason of the rather large shunting capacity between the electrodes thereof and frequency variations resulting from changes in said capacity during tube operation. The response of such a low Q circuit is illustrated diagrammatically in Fig. 3, which is a curve illustrating the action of a diode connected to a resonant input circuit. In this case, since substantial current flows through the tube, resistive loading components are present and the Q of the circuit is lowered greatly, resulting in a curve of the type shown in Fig. 3. It can be seen that this curve is not as steep as that shown in Fig. 2 and is also not as linear; therefore, variations in the operating point, which is indicated as normally being 14 megacycles, falling at the point 42 would result in non-linear, relatively weak amplitude signals occurring in response to frequency variation. Furthermore, the band width is approximately 200 kilocycles in the examples shown, resulting in much greater undesirable noise and a lower signal-to-noise ratio.

The condenser 38 acts to bypass carrier frequency to ground, and the carrier frequency choke 40 acts to prevent the passage of such carrier frequency through the source of positive potential. The carrier frequency choke 30 acts to prevent the passage of carrier frequency through the primary winding 32 of the audio transformer 33, while the condenser 3| allows passage of audio frequency therethrough and through the primary winding 32 of the audio frequency transformer 33. For purposes of compactness, it is desirable that the tubes 3 and 23 be of the double triode type, wherein both tubes are contained in the same envelope. However, this arrangement is not at all necessary, and I do not intend to limit myself to such an arrangement. It should also be noted that while I have shown the variable capacitor comprising the condenser microphone 2 I, alone shunting the coil 20, I also contemplate having an additional fixed shunting condenser thereacross in certain forms of this invention, whereby the capacitive change of the condenser 2! in response to auditory input will be relatively small in comparison to the capacitance of the fixed shunting condenser.

It should be noted that prior art condenser microphones, by reason of a large capacity used therein, had both a low frequency electrical cutoff, resulting from the increased capacity reactance of such frequencies, and non-linear high frequency response resulting from the capacitive shunting effect as seen from the amplifier. This is not true of the apparatus of the present invention, which is linear in its response from extremely low frequencies to extremely high frequencies. In addition, the microphone of this invention may place the diaphragm and plate very close together, of the order of /4 mil apart, in comparison with the spacing of prior art condenser microphones, which was of the order of 2 or 3 mils. This may be achieved because no large voltage exists across the microphone of the present invention, such as is employed in prior art condenser microphones. Therefore, a very large capacitive change will result from relatively small acoustic input to a very small diaphragm, as employed in the present invention. This produces a much higher signal output for a given acoustic input than prior art microphones. By reason of the hereinabove described construction of the present invention, a diaphragm having an effective moving diameter of produces an output of 20-30 decibels greater than prior art condenser microphones having diaphragms of the order of 3" in diameter.

I also contemplate employing the apparatus of this invention in a directional microphone by combining it with a pressure gradient-sensitive type of microphone or velocity type of microphone, whereby acoustic waves from one direction are in phase and additive, and acoustic waves from the other direction are 180 out of phase and cancel each other. It is particularly adapted for this, because the small size of my condenser microphone makes it possible to place the two microphone elements of my directional microphone comprising my condenser microphone and a stretched ribbon, for example, physically very close together, thus causing the directional effect to extend into the very high frequency range, which no present microphone will do, prior art directional microphones being directional in only the low and medium frequency ranges.

It should be noted that this invention my be very readily adapted for use as a phonograph pick-up. In such cases, the moving needle or stylus, or an armature or other element connected l thereto, may be placed adjacent fixed capacitive plates or a single plate, for causing capacitive variations in response to the vibrations of the phonograph needle caused by the variations in the recorded grooves of the record. In this case, the moving element and the fixed capacitive plate or plates will comprise the variable capacitance 2| of the invention.

It should be noted that the apparatus of the present invention may also be employed in phase modulation. For example, the tuned resonant input circuit l9 may be tuned to the carrier frequency exactly, if desired, in which case capacitive variations of the microphone 2| result in the circuit l9 alternately presenting itself as an effective inductance and as an effective capacitance, thus phase modulating the carrier frequency. The cathode output circuit may be dispensed with entirely, if desired in this case, and a suitable tanl: circuit employed in the plate circuit, acting as a frequency multiplier. Several successive stages of frequency multiplication may be employed, if desired, and also a limiter for canceling out any amplitude modulation present may be employed. Suitable means for compensating at a rate inversely proportional to audio frequency may be employed for rendering modulation width virtually constant for low and high frequencies.

This invention is also adapted to be used in a slightly modified form, wherein the tuned resonant circuit, indicated at l9, comprises an inductance shunted by the auditorily variable condenser, Which is in the form of a pressure-sensitive diaphragm and adjacent condenser plate, and may be separated from the balance of the modulator and the oscillation-generating means by a very considerable distance, if desired. This is highly desirable in motion picture sound recording, for example, since a long, hollow, aluminum alloy tube comprising a movable boom may carry only the pressure-sensitive diaphragm and adjacent condenser plate and a shunting coil, at the movable end thereof, which is adapted to be placed in the region adjacent the source of the sound which is to be recorded. The aluminum alloy tubing may contain a suitable wire means therein, thus comprising a coaxial cable, which at its rear end may be electromagnetically linked to the input circuit of an amplifier tube similar to that shown at 23 in Fig. 1. Thus, the entire transducer comprising the oscillation generator and the modulator need not be mounted at the point Where the sound is picked up but may be suitably, fixedly mounted and connected by said coaxial cable means to the movably mounted microphone unit shunted by an inductance carried at the far end of the aluminum alloy tube. This further reduces the Weight and size of the actual microphone unit carried at the end of the boom and makes it possible to increase the size of said unit to such a point that the output of the transducer may be directly used to control a light valve for motion picture film sound track recording without any further stages of amplification.

In addition, the acoustically-responsive condenser may be arranged for varying the output thereof in response to a given auditory input thereto by arranging the diaphragm and condenser plate for adjusting the distance therebetween or arranging the diaphragm for adjusting the tension thereon.

Numerous modifications and variations of this invention are possible within the scope thereof and will occur to those skilled in the art. For example, any type of oscillation generator may be employed, such as a negative resistance oscillator or a feed back oscillator, etc. The arrangement of the modulator may be varied within wide limits. Under some circumstances, the vacuum tube in the modulator may be dispensed with. The LG product of tuned circuit means may be varied in response to mechanical input in any desired manner. For example, capacitance need not necessarily be varied but the inductance may be varied in response to mechanical movement, such as, for example, moving a ferromagnetic core with respect thereto or in any other desired manner. The tuned circuit may be a series tuned circuit instead of a parallel tuned circuit, if desired, somewhat similar to that shown in the series resonant input to the second stage of the amplifier shown on page 336 of Fundamentals of Engineering Electronics, by William G. Dow, published by John Wiley & Sons, Inc., or modifications thereof which are obvious. All such modifications are intended to be comprehended herein.

Throughout this application, the term low frequency is understood to mean in the region f the audio frequency range or below, and the term high frequency is understood to mean above the audio frequency range up to and including ultra high frequencies.

The example described and illustrated herein are exemplary only and are not intended to limit the scope of this invention, which is to be interpreted in the light of the appended claims only.

I claim:

1. An acousto-electric transducer comprising: a source of electric oscillations of a predetermined high frequency; a modulator including a resonant circuit having an inductor and a capacitor connected in parallel, said inductor being adjustably electromagnetically coupled to said source, said capacitor constituting a condenser microphone, the quiescent resonant frequency of said circuit being slightly different from the frequency of said source, said circuit being arranged to modulate said high frequency oscillations by low frequency oscillations in accordance with auditory input to said microphone; a virtually infinite input impedance demodulator electron tube having a cathode, a control grid and an anode, said resonant circuit being eifectively connected between said control grid and ground;

10 a source of positive potential connected to said anode; a cathode resistor between said cathode and ground, said cathode resistor being of such value that the maximum amplitude of the high frequency oscillations fed to said tube falls on a virtually linear central portion of the Ip vs. Eg curve of said tube, whereby the tube will operate class A with respect to low frequency modulating oscillations; a radio frequency capacitor across sa' cathode resistor; and an output circuit cond across said cathode resistor and said radio equency capacitor.

An acousto-electric transducer comprising: a of electrical oscillations of a predeterhigh frequency; a modulator including a pa allel resonant circuit adjustably coupled to sa source and means responsive to auditory inl5 varying the resonant frequency of said resonant circuit, the quiescent resonant frequency of said circuit being slightly different from the frequency of said source, said circuit being arranged to modulate said high frequency oscillations by low frequency oscillations in accordance with input to said means; a virtually infinite input impedance demodulator electron tube having a cathode, a control grid and an anode, said resonant circuit being effectively connected between said control grid and ground; a source of positive potential connected to said anode; a cathode resistor between said cathode and ground, said cathode resistor being of such a value that the maximum amplitude of the high frequency oscillations fed to said tube falls on a virtually linear central portion of the Ip vs. E; curve of said tube, whereby the tube will operate class A with respect to low frequency modulating oscillations; a radio frequency capacitor across said cathode resistor; and an output circuit connected across said cathode resistor and capacitor.

STANLEY M. CHURCH.

References Cited in the file of this patent UNITED STATES PATENTS 

